Protection of permselective diaphragm during electrolysis



April l5, 1969 M. s. K|RCHER ET Ax. 3,438,879

PROTECTION OF PERMSBLECTIVE DIAPHRAGM DURING ELECTROLYSIS Filed July 3l. 1967 CATHODE Tou EQF

1 RlNE O IN IN SOLUTION OUT United States Patent O 3,438,879 PROTECTION F PERMSELECTIVE DIAPHRAGM DURING ELECTROLYSIS Morton S. Kircher, Vancouver, British Columbia, Canada, and George T. Miller, Lewiston, N.Y., assignors to Hooker Chemical Corporation, Niagara, Falls, N Y., a corporation of New York Continuation-impart of application Ser. No. 250,292, Jan. 9, 1963. This application July 31, 1967, Ser. No. 664,587

Int. Cl. C0111 1/06; C01b 7/06; B01h 3/10 U.S. Cl. 204--95 13 Claims ABSTRACT OF THE DISCLOSURE In the electrolytic decomposition of an aqueous solution of a chemical compound in an electrolytic cell having a permselective diaphragm adjacent an anode, in contact with anolyte, the method of extending the useful life of the said permselective diaphragm during electrolysis, which comprises interposing a chemically resistant porous protective barrier between the said anode and the said permselective diaphragm, thereby forming a buffer zone between the diaphragm and the anolyte in the anode compartment and introducing feed for the anode compartment into the said buffer zone between the permeselctive diaphragm and the protective barrier, so that the anolyte feed percolates from the buffer zone through the porous barrier into the anode compartment, thereby keeping anode electrolysis products away from the face of the permselective diaphragm. A tetrauoropolyethylene screen is preferred.

This is a continuation-impart of Ser. No. 250,292, filed Jan. 9, 1963, now abandoned.

This invention relates to improvements in the electrolytic decomposition of chemical compounds and the production of useful products therefrom. Further, this invention relates to a novel method for extending the life of permselective diaphragms used in electrolytic cells, especially in the electrolysis of alkali metal chloride to produce chlorine, hydrogen and alkali metal hydroxide. More particularly7 this invention involves a method and apparatus for the protection of the permselective diaphragm adjacent the anode against the anolyte of the electrolytic cell, so that the life of the diaphragm is greatly extended and the overall cell current eiciency is improved.

Permselective diaphragms are characterized by their substantial impermeability to liquids and by their selective permeability to ions of one charge and substantial irnpermeability to ions of the opposite charge when wet with an electrolyte and under the influence of an electrical current. Permselective diaphragms which selectively permit the passage of anions are designated as anionic; those which selectively permit the passage of cations are designated as cationic. The practical utilization of permselective diaphragms in elecrolytic cells for the electrolytic decomposition of chemical compounds, and the separate recovery of the products resulting therefrom, depend upon the presence of a polar medium, generally water, being present in the pores of the diaphragm; that is, to say, permselective diaphragms must be wet with electrolyte in order to function in accordance with this invention. Such diaphragms and certain methods and apparatus for employing them in the electrolytic decomposition of chemical compounds are more fully set forth in U.S. Patent 2,967,807, in which there is disclosed the fundamental concept of simultaneously controlling both molecular and ionic migration during the electrolytic decomposition of chemical compounds between the electrodes of an electrolytic cell, by electrolyzing the chemical com- ICC pound in a cell separated into electrode compartments, by one or more permselective diaphragms. By this means, the electrical current introduced during electrolysis will be carried through the permselective diaphragm substantially by ions of one charge, because permselective diaphragms inhibit the passage of the ions of opposite charge. For example, when sodium hydroxide and elemental chlorine are produced by the electrolytic decomposition of an aqueous solution of sodium chloride brine in an electrolytic cell in which the brine in the anolyte at the anode is separated from the caustic soda produced at the cathode, by a cationic permselective diaphragm, the diaphragm operates in two distinct manners. During electrolysis, sodium ions migrate from the anolyte to the cathode and upon reaching the cationic permselective diaphragm, which divides the cell into separate anode and cathode compartments, are permitted passage through the diaphragm from active point to active point in the pores of the cationic permselective diaphragm because such diaphragm selectively permits passage of such sodium cations While selectively prohibiting the passage of anions such as chloride ions and hydroxyl ions, thereby enabling the isolation and separate recovery of sodium hydroxide in the cathode compartment, and permits an increase in NaOH concentration in the catholyte with continued electrolysis.

One of the major problems to be overcome in making such electrolytic cells commercially suitable for the continuous electrolysis of alkali metal chloride brines, has been in extending the life of the permselective diaphragms. Attack of the cationic permselective diaphragm facing the anode by the anolyte has been found to render the life of even the most chemically resistant permselective diaphragms relatively short.

The anolyte of electrolyzed alkali metal chloride is very corrosive. In addition to sodium and chlorine ions, it contains decomposition products of. brine electrolysis, such as elemental chlorine, elemental oxygen, chlorate ion, hypochlorite ion, chlorine dioxide, perchlorate ion, etc., all at elevated temperature. The elemental chlorine gas in the anolyte in both the dissolved and gas phases, appears to be one of the major causes of diaphragm failure.

Moreover, since the permselective diaphragm is used to separate two different electrolytes, it should not only be chemically resistant to the anolyte but it should also be resistant to the electrolyte it separates from the anolyte, which would include alkali metal hydroxide.

Over the few years since the idea was conceived to use a permselective diaphragm in the commercial electrolysis of alkali metal chloride brines, many different cationic permselective diaphragms have been developed. For instance, the cationic permselective diaphragms employed in the electrolytic decomposition of alkali metal chloride brines can be constructed using ion exchange resins which have been formed into continuous thin sheets. Successful diaphragms for use in electrolysis of alkali chloride brines have been constructed using cation exchange resins of an Amberlite type which have been formed into continuous sheets. These diaphragms are described in United States Patents 2,681,319 and 2,681,320, and sold as Amberplex type membranes. Such sheets are continuous, non-porous, self-supporting, pliable, permselective membranes or pellicles which comprise a matrix-such as a synthetic hydrocarbon type plastic, or vulcanized, natural or synthetic rubber or polyethylene or polyisobutylene, or polyvinyl chloride, or copolymers of vinyl chloride and vinyl esters of lower aliphatic acids--having distributed intimately and uniformly therein particles of an insoluble, infusible ion-exchange resin, said particles being of such a size as to pass through a No. 50 sieve (United States Standard Sieve Series) and being present in said diaphragm in an amount equal to twenty-tive to seventy-tive percent of the total weight of the diaphragm. The degree of pefection attainable in operation when using such diaphragms in electrolysis is among other things a function of its tightness, or the number of macropores or leakage points occurring between the resin particles in the sheet or fiber.

Particularly suitable diaphragms for electrolysis of alkali metal chloride brine are those covered by U.S. Patent Nos. 2,978,393, 2,978,401 and 2,978,402. But even when using these diaphragms in accordance with the teaching of the prior art, the continuous operating life of them is short because of the high attack rates encountered during electrolysis.

We have found that the life of the permselective diaphragm facing the anode in an electrolyte cell for the electrolysis of an aqueous solution of a chemical compound, such as alkali metal chloride brine, can be greatly increased by protecting it against deterioration by the improvement which comprises interposing a chemically resistant porous screen or barrier between the anode and the permselective diaphragm facing the anode, as a protective barrier and thereby forming a buffer zone or compartment between the anolyte in contact with the anode in the anode compartment and the permselective membrane facing the anode, and introducing feed for the anode compartment, e.g., the brine, into the said buffer zone between the protective barrier and the permselective diaphragm, preferably at the bottom or lower portion, so that the brine percolates from the buffer zone through the porous barrier toward the anode, and preferably also sweeps up and out of the upper portion of the cell compartment, thereby keeping to a minimum the concentration of anode electrolysis products at the face of the permselective diaphragm. When operating under our preferred conditions, chemical attack of the diaphragm is reduced to 1/200 or less of what it would be without the screen.

We have found further -that for some unknown reason, there are indications that the voltage drop across the protective barrier is less than the voltage drop across a brine gap of the same size.

A preferred porous protective barrier is fabricated from polytetrauoroethylene, sold under the trademark Teflon, such as in a woven sheet cloth, screen, sintered plate or other suitable form, but effective barriers have also been fabricated from other chemically resistant porous screening materials including glass and synthetic organic plastics, such as Saran (a copolymer of vinylidene chloride and vinyl chloride), Dynel (a coploymer of acrylonitrile and vinyl chloride), and polypropylene. Of these, it is preferred to employ types` which are non-conductive or susbtantially non-conductive.

The electrodes to be used in the method of this invention are those commonly used in the elec-trolytic decomposition of aqueous solutions. For the anode, carbonaceous materials, such as graphite and impregnated carbonaceous materials are to be used. For the cathode, steel, iron, or some other material resistant to the reagents in the cell, are to be used.

In order that this invention may be more readily understood it is described with specific reference to certain preferred embodiments thereof as shown in the figure, which is a diagrammatic sketch illustrating the electrolysis of an aqueous solution of sodium chloride at the anode land the carbonation of the solution in the center compartment of our three compartment electrolytic cell with carbon dioxide, whereby hydrogen, chlorine, sodium carbonate and `caustic soda are separately produced and recovered. However, the invention is not to be construed as being limited thereto, as will be apparent from the modifications discussed hereinafter, except as defined in the appended claims.

Referring to the figure, which is a diagrammatic sketch illustrating specific embodiments of this invention: the electrolytic Cell comprises a vessel 1 separated into an anode compartment 2 a center compartment 3 and a cathode compartment 4 by two cationic permselective diaphragms S and 6. The anode compartment is separated into two zones by a porous protective barrier 21. The anode zone contains an anode 7 in contact with sodium chloride brine 8.

The anode zone of the anode compartment also contains a brine outlet 10 and a chlorine outlet 11. The buffer zone of the anode compartment contains a brine inlet 9 and a brine sweep outlet 22. The cathode compartment contains a cathode 12 in contact with water or caustic 13. The cathode compartment also contains a water inlet 14, a caustic outlet 15 and a hydrogen outlet 16. The center compartment 3 is provided with inlets 17 for introducing water 18 and carbon dioxide 19 into the center compartment of the cell, which is positioned between the cationic permselective diaphragm facing the anode and cationic permselective diaphragm facing the cathode, -designated herein as 5 and 6, respectively. The center `compartment is also provided with an outlet 20 for removing the sodium carbonate solution which is produced under the conditions maintained during electrolysis.

In accordance with a preferred form of our invention, the anode 7 in the anode compartment 2 is separated from the cationic permselective diaphragm 5 by a porous protective barrier 21 positioned in the anode compartment so that the brine inlet 9 is between the protective barrier 21 and the cationic permselective diaphragm 5 and preferably at or near the bottom of the cell compartment, so that during operation of the cell, the incoming brine both percolates through the protective barrier 21 toward the anode 7 and also sweeps up the cell along the cationic permselective diaphragm and out the sweep brine outlet 22, thereby keeping the anode decomposition products of electrolysis away from the cationic permselective membrane 5 and preventing the collection of gaseous or solid materials at the surface of the membrane 5. In addition, the electrolytic cell may contain any other necessary assessories for the given electrolysis.

In accordance with this invention a saturated solution of sodium chloride brine 8 is fed into the anode compartment 2 of the cell 1 at a point between the protective barrier 21 and the cationic permselective diaphragm 5, preferably at or near the bottom, through brine inlet 9. The brine then sweeps up the face of the diaphragm 5 and percolates through the porous screen 21 into the anode side of the anode compartment 2. In this way, the anode products of electrolysis are both swept away from the surface of the diaphragm 5 and kept back from diaphragm by the countercurrent flowof brine through the screen 21.

As a preferred embodiment of our invention, an excess of brine can be fed in at inlet 9 and that amount of electrolyte which does not percolate through the barrier 21 can be removed through the sweep brine outlet 22. In this way, the sweep effect along the face of the permselective diaphragm 5 is enhanced. Depleted brine is removed from the anode compartment through brine outlet 10. Chlorine produced is removed Vfrom the cell through outlet 11. A starting solution of sodium carbonate is introduced into compartment 3 in order to lower voltage at the start of electrolysis, then water 18 and carbon dioxide 19 are fed into the center compartment of the cell through inlet 17. The sodium carbonate solution produced in the center compartment during the electrolysis is removed from the cell outlet 20. The caustic soda produced in the cathode compartment 4 is removed through outlet 15. Hydrogen which is also produced in the cathode compartment is removed through outlet 16. Water can be introduced into the cathode compartment through inlet 14 in predetermined amounts in order to produce caustic soda of the desired concentration. In an alternative mode of operation the cathode compartment may acquire water through the diaphragm 6 by electroosmosis, especially if the particular diaphragms employed have minor leakage points. In such case the amount of water introduced through inlet 14 will be correspondingly minimized or entirely eliminated.

Another alternative mode of operation involves introducing the carbon dioxide (which causes the formation of sodium carbonate, the fourth product of the cell), into the solution produced in the center compartment of the cell, at a point removed from the electrolytic cell, such as at a point subsequent to outlet and circulating the sodium carbonate solution so produced back into the center compartment to favor maintaining a substantially uniform concentration of sodium carbonate throughout the solution and adding water as necessary to do this. In this way, the precent permselectivity of permselective diaphragms at a given caustic concentration can be increased by approximately the amount of permselectivity lost due to back migration.

When the cell design is such as to require a large sheetlike permselective diaphragm to be spaced by a gaskettype spacer we have found i-t desirable to reinforce the diaphragm by placing `a grid against the face of it to buckling. The grid may consist of a rigid screen or similar type structure of suitable material of construction disposed across the entire surface of the diaphragm. If a diaphragm buckles inwardly toward the center compartment, due to pressure from an adjacent compartment, a grid may be positioned in contact with the face of the diaphragm in said center compartment and is adapted to prevent buckling and contact between the diaphragms. If a diaphragm buckles outwardly toward an adjacent compartment due to pressure from the center compartment, a grid may be positioned in the adjacent compartment, in contact with the face of the diaphragm and is adapted to prevent buckling and contact with the electrode if the next adjacent compartment is an electrode compartment.

It is to be understood that the porous protective barrier 21 can be `fabricated from any suitable material which is chemically resistant to the anolyte. As stated above, we prefer to use a screen of closely woven Teflon cloth. However, other porous materials have also been found to be effective in reducing the attack on the diaphragm and extending its life. Among these are asbestos, glass, polyethylene, Saran and Dynel. Other chemically resistant porous materials, such as: polypropylene, halogenated organic polymers including the various polymers and copolymers based -on vinyl chloride and polymonochlorotriuoroethylene sold under the trademark Kel-F, may also be used.

The protective barrier can be fabricated in a number of ways. As previously mentioned, it can be made from woven fibers. It can also lbe fabricated from sintered particles or felted fibers. It can be made from foamed plastic materials or from leaching or solvent displacement of a phase of a 4heterogeneous structure. The invention is not limited to the method of fabrication.

The porous protective barrier should preferably have a relatively small pore size. A typical porosity of Teflon screen was about a 40 micron pore size having an air porosity of approxi-mately three cubic feet per square foot of area per 0.5 inch of water pressure differential.

The porous protective barrier should be spaced from the anode and from the permselective diaphragm in order to permit substantially uniform percolation of -feed brine through the entire area of the protective barrier. Failure to achieve such uniform flow may result in a build-up of materials within the porous barrier, passage of chlorine gas into the protected zone, chemical attack of the diaphragm, etc. Such spacing may include using chemically resistant reinforcing and/or spacing means along one or both of the surfaces of the porous barrier. Such means can be glass rods extending from opposite edges of the spacing frame adjacent to the porous barrier. They can be ribbing material or screening, either separate or woven into the barrier itself. They can be chemically inert projections (integral or non-integral) or gasketing strips, etc., afxed to one or more of the surfaces of the anode, porous barrier or diaphragm, to maintain proper spacing so as to permit anolyte and brine flow across the surfaces of the anode, land diaphragm, respectively. The spacing means may be the same as or different yfrom any reinforcing means for the diaphragm, as referred to above.

It is also to be understood that the invention embraces cells for the electrolytic decomposition of alkali metal chlorides having only one permselective membrance or producing'only alkali metal hydroxide and not sodium carbonate, or some other material as an additional product of electrolysis, or having a central compartment between the permselective diaphragm facing the anode and the cathode.

It has `been found that for the most effective operation, the brine being introduced into the space between the -porous protective barrier and the permselective diaphragm facing the anode should be introduced in such a way as to sweep clea-r all portions of the diaphragms exposed anode surface. This is accomplished effectively by feeding the brine at or near the bottom of the diaphragms anode face, preferably along the bottom. However, the brine can be fed in at other points around the diaphragms periphery, either alone or in conjunction with feeding along the bottom. In this way, the possibility of having dead spots where corrosive solids, chlorinated brine, chlorine gas, or other corrosive products of electrolysis, could accumulate on the diaphragms anode face, is reduced.

It has also been found highly desirable to feed excess feed brine under a sucient head or pressure so that the brine not only percolates through the porous barrier in a direction toward the anode, thereby retarding the ow of anode electrolysis products toward the diaphragms face, but also sweeps out `of the cell at the top, thereby further minimizing the possibility of dead spots, especially at or near the upper -portion of the diaphragms anode surface. The amount of excess brine to be used can `be varied -within wide limits and depends to a great extent on the cell design and operation. Use of the Teon cloth porous barrier and an increase in the brine feed rate -by a factor of about three, was found to give an improved sweep of the anode surface of the `diaphragm and a substantially decreased chemical attack rate on the diaphragm over that obtainable by use of the porous barrier without the -sweep brine system. The brine feed rate to use is ultimately a matter of economics. A cell having a lbarrier of relatively high porosity will benefit by the use of high `brine sweep rates as compared with a cell having a barrier of relatively low porosity.

It is also to be understood that in addition to the introduction of the brine into the anode compartment between the permselective diaphragm and the porous protective barrier, the brine can also be fed `concurrently into the anode compartment on the anode side of the protective barrier.

Among the preferred chemical compounds which can be electrolyzed in accordance with the method of this invention are solutions of soluble inorganic salts such as the aqueous solutions of the alkali and alkaline earth metal bromdes, chlorides, sulfates, acid `sultes, etc. The alkali metal salts such as lithium and potassium which are very soluble and electrolyzable in water can be successfully electrolyzed in the same manner as the sodium salts depicted above. For example, the electrolysis of sodium sulfate solutions are described in U.S. Patent 2,967,807 in addition to sodium chloride; and in 2,967,806 the electrolysis of water-soluble salts of or-ganic acids is described.

Chemical reagents other than carbon dioxide may be used in the central compartment of the preferred cell shown in the gure. The reagent selected will depend upon the salt to be electrolyzed and the product `which is desired. Inorganic and organic acids or substances capable `of reacting with bases such as SO2, HCl, H28, H3PO4, acetic acid, benzoic acid, etc., may all be used to react with the migrating hydroxyl ions coming from the cathode compartment through the imperfect cationic permselective membrances. Similarly, where it is desired to add a lbase into 'an additional `compartment to prevent fast moving hydrogen ions fro-rn yreducing the current eiciency by being lost because of leakage in the anionic permselective membrance, the choice of this base will depend upon the salt to be electrolyzed and the product which is desire-d, as above. Alkali metal hydroxides or carbonates are among the preferred compounds to be used. In either case the product need not be water solub'le, but this is desirable especially where the reagent is introduced directly into the cell compartment. The metallic salts corresponding to the Iacid or 1base introduced into the center `compartment of the diaphragm structure of this invention will of cou-rse be produced by the reaction between the acid or base and the hydroxyl or hydrogen ions. For exa-mple, when the reagent added is sulfur dioxide the additional product produced by the cell lwill be sodium sulfite; by controlling the proportion of 'chemical reactant introduced, sodium bisulte may be produced.

The operating conditions such as current density, applied voltage, temperature, feed and product concentrations, various additives and other conditions are familiar to those skilled in the art are considered not to be limiting the scope of this invention.

The following examples are given to further illustrate this invention and should not be construed as limiting except as defined in the appended claims.

Example l An electrolytic cell was constructed of a hard rubber body in a manner after that shown in the figure. The anode was a block of 6" x 6 X 1% graphite. The cathode was 6 mesh, 0.16 inch diameter hot-rolled wire screen (mesh) used as standard cathode material in commercial Hooker Type S deposited diaphragm type electrolytic cells. Sandwiched between the anode and the cathode was an assembly of the following elements (beginning from the anoe side of the assembly): (1) a 1/16 inch rubber gasket, used as a spacer, (2) a closely woven Teflon cloth used as a protective barrier, (3) a 1A inch gum rubber gasket used as a spacer, (4) a cationic permselective diaphragm made in accordance with Example 3 of U.S. Patent 2,978,402 designated as Hooker-31 diaphragm, (5) a 1/16 inch rubber gasket, used as a spacer, (6) a steel frame having therein steel screening, used to support the diaphragms facing each side of the steel screen, (7) a 1/16 inch rubber gasket and (8) a cationic permselective membrane of Amberplex C-l under U.S. Patent 2,681,320. The cathode end section had a ange which was bolted to a hard rubber anode compartment, holding in place the above assembly sandwiched between them. The overall dimensions of the assembled cell were 8" x 8" x 4".

Potassium chloride brine was introduced into the space between the Teflon cloth and the Hooker-31 diaphragm at the bottom through an opening in the gum rubber gasket. Eiuent brine was removed from an outlet through the hard rubber of the cell. Potassium carbonate solution was introduced into the bottom of the center compartment formed by the two permselective diaphragms through au inlet and removed from the top of the said center compartment (which during electrolysis increased in its volt content). Chlorine gas was removed from the anode compartment and gaseous hydrogen was removed from the cathode compartment by means of gas outlets therein, and KOH solution was removed from the cathode compartment through outlet means therein. After 18 days of electrolytic decomposition activity at a current density of about amperes/sq. foot, the cell was taken apart. No measurable chemical attack occurred on the Hooker- 31 diaphragm; whereas appreciable attack occurred in other runs under comparable conditions where a protective barrier cloth was not used.

8 Example 2 The cell design of Example l was enlarged in its crosssectional area from six inches by six inches to 17% inches wide by 45 inches high. Brine sweep outlet means were provided in the top part of the rubber gasket separating the T eon screen from the Hooker-31 diaphragm.

During operation of this pilot plant size cell, the brine feed rate was maintained at 543 cc/min. of 26.9 percent KCl, and the sweep brine efliuent rate was maintained at 395 cc/min. of 26.0 percent KCl, while maintaining a current density of about amperes/ft.2 across the diaphragms.

Under these conditions, as long as the diaphragm face is kept free of dead areas of poor circulation, etc., the chemical attack rate of the anode face of the diaphragm, even after 40 days of continuous operation, is nil. Without the use of the synthetic organic plastic barrier, the attack rate on the anode face of the permselective diaphragm adjacent to the anode in this cell under comparable operating conditions would have been 3 to 5 mils/ day.

Although we have described our invention with respect to certain specific embodiments thereof we do not intend to be limited thereto except as detned in the following claims.

What is claimed is:

1. In the electrolytic decomposition of an aqueous solution of a chemical compound in an electrolytic cell having a permselective diaphragm adjacent to and separated from a carbonaceous anode, in contact with anolyte, the method of extending the useful life of the said permselective diaphragm during electrolysis, which comprises interposing a chemically resistant porous protective barrier between the said anode and the said permselective diaphragm, thereby forming a buffer zone between the diaphragm and the anolyte in the anode compartment, and introducing feed for the anode compartment into the said buffer zone between the permselective diaphragm and the protective barrier, so that the anolyte feed percolates from the buffer Zone through the porous barrier into the anode compartment, thereby keeping anode electrolysis products away from the face of the permselective diaphragm.

2. The improvement of claim 1 in which the porous protective barrier is of synthetic organic polymeric plastic material.

3. The improvement of claim 1 wherein the porous protective barrier is fabricated from polytetrafluoroethylene.

4. The improvement of claim 1 wherein the porous protective barrier is woven polytetrafluoroethylene cloth.

5. The improvement of claim 1 wherein the porous protective barrier is fabricated of a copolymer of vinylidene chloride and vinyl chloride.

6. The improvement of claim 1 wherein the porous protective barrier is fabricated of a copolymer of acrylonitrile and vinyl chloride.

7. The improvement of claim 1 wherein the porous protective barrier is fabricated of polypropylene.

8. The improvement of claim 1 wherein the porous protective barrier is fabricated from glass cloth.

9. The improvement of claim 1 in which the porous protective barrier is fabricated from polyethylene.

10. The improvement according to claim 1 wherein the feed for the anode compartment is introduced into the butter zone under pressure greater than in the anode compartment and swept through the buffer zone so that the material in the buffer zone percolates from the buffer zone through the porous barrier toward the anode and also sweeps out the upper portion of the buffer zone itself.

11. The improvement according to claim 1 wherein the aqueous solution being electrolyzed is au alkali metal halide.

12. The improvement of claim 11 wherein the alkali metal halide is potassium chloride.

13. The method of producing the electrolytic decomposition of an alkali metal halide brine comprising passing said brine into a four compartment electrolytic cell having an anode compartment receiving liquid electrolyte from a next adjacent anode buffer zone compartment through a chemically resistant porous protective barrier and a carbonaceous anode in contact with said electrolyte and discharging halogen, and having a cathode compartment receiving cations from a next adjacent compartment through a cation permselective diaphragm, and a cathode in contact with electrolyte and discharging alkali hydroxide, the anode butter zone compartment being separated from the compartment adjacent the cathode compartment by a cation permselective diaphragm, maintaining the diaphragms wet with elec trolyte, impressing a decomposition voltage across said electrodes, feeding the brine into the buffer zone compartment under a pressure greater than in the anode compartment and sweeping the brine through the buffer zone so that the brine in the buffer zone percolates from the buffer zone through the said porous barrier toward the anode and also sweeps out the upper portion of the buier zone itself.

References Cited UNITED STATES PATENTS 1,126,627 1/1915 Gaus 204-98 2,723,229 11/1955 Bodarner 204-98 2,944,956 7/1960 Blue et al. 204-266 2,967,807 l/l961 Osborne et al. 204-98 3,222,267 12/ 1965 Tirrel et al. 204--98 JOHN H. MARK, Primary Examiner.

D. R. JORDAN, Assistant Examiner.

U.S. Cl. X.R. 

